you can't really counter Nasus, unless your junlge camps him, that's about the only real counter, within minutes he'll turn into a monster and there's no real way to stop him from farming his Q, and he has enough sustain to outlast the pokes coming from other champs, they'll just end up out of mana or missing cs trying to harass you...surviving the first 5 or so levels is crucial
& Nasus absolutely crushes adc lanes, if they don't have a stun or an escape you will come out winning every trade after you get tabi's, just wither the adc and go to town on him with aa-Q-aa combos, once you finish off a phage they will not wanna be anywhere near you, and if they are they just die